1. Field of the Invention
This invention relates generally to the field of inverter control and specifically to a sine wave generator for a motor controller.
2. Description of the Related Art
It has become increasingly desirable in recent years to power motors from direct current sources. Most DC motors require brushes which are susceptible to dirt and require maintenance. Permanent magnet type motors do not necessarily use brushes, but magnet technology has been unable to keep pace with the demand for large and inexpensive motors. Thus, it is common to invert direct current to alternating current which is used to power an AC motor such as an induction or synchronous type motor.
Typically, an inverter controls application of a DC voltage to the motor through an array of switches such as transistors which are controlled to create an alternating current. As is known in the art, one or more control signals are used to control the switches. The control signals most commonly are derived from a sine wave. In many cases, the sine wave is a digital word used to generate a pulse width modulated signal which, in turn, controls the transistors.
Most commonly, the sine wave is generated by a microprocessor which refers to a look-up table of sine values and multiplies those values as needed. However, a microprocessor can be relatively expensive. Another method involves directly calculating sine wave values, however, such calculations are not merely arithmetic and, therefore, require complex circuitry or a microprocessor. In any case, sine values must be multiplied to increase or decrease the magnitude of a control signal, for example, for acceleration of a motor.
It would be desirable to generate a digital sine wave signal for an inverter without complex or expensive circuitry. Variable amplitude and variable frequency sine waves can be generated by using simple arithmetic calculations.